Refrigeration and air conditioning in buildings accounts for approximately 8.5 quads of U.S. primary energy consumption. Most conventional air conditioners and refrigerators achieve cooling through a mechanical vapor compression cycle (VCC). These systems suffer from relatively low efficiency. Typically, the vapor compression refrigerators (VCR) have a coefficient of performance (COP) of 2-4. The COP can be expressed as COP=Qc/W, where Qc is the heat extracted from the cold end and W is the work required. There does not appear to be any economically viable avenue to markedly improve the efficiency of these VCC systems. Further, air conditioning is a major contributor to electric utility peak loads, which leads to high generation costs and generally use inefficient and polluting generation mechanisms. Peak loads are also a major factor in poor grid reliability. A related problem with today's VCC cooling technology is the adverse environmental impact of the refrigerant gases employed. Even though the hydrofluorocarbon (HFC) refrigerants in the current cooling system are relatively safe for the ozone layer, they are strong green house gases.
These factors necessitate a search for new approaches to increase the energy efficiency of these cooling technologies which are meanwhile environmentally friendly and low cost.
One approach that has gained much attention as a solid state refrigeration technique is the magnetocaloric effect (MCE), which is a magnetic field induced isothermal entropy change and adiabatic temperature change in a magnetic material. (K. A. Gschneidner Jr, V. K. Pecharsky, and A. O. Tsokol, Rep. Prog. Phys. 2005, 68, 1479.) Isothermal entropy change (ΔS) from MCE is a result of the change of molecular ordering on a micro or macro scale under the application of a magnetic field or removal of a magnetic field at a constant temperature of the surrounding environment. ΔS is related to the heat Q extracted from the cold end at a temperature T as Q=TΔS. The adiabatic temperature change (ΔT) is the change of temperature of the material under the application or removal of a magnetic field without exchanging heat from the surrounding environment. Essentially this effect exists in magnetic materials and it has been found that by operating above the ferromagnetic-paramagnetic transition, a phase transition can be induced by applying or removing a magnetic field, which leads to a large MCE (ΔT>10° C. and ΔS>30 J/(kgK)).
At the current time, the MCE is used only in high end niche applications due to certain short comings such as very high magnetic fields (>5 Tesla), low cooling efficiency (high energy loss to generate high magnetic field).
Accordingly, a need exists for improved and additional techniques and devices that can transfer heat with energy efficiency and in an environmentally friendly manner.